In radiological practice, it is common to display different exposures pertaining to a patient study in a predefined format. This feature is known as a hanging protocol. In film-based operation, it means that the radiologist or operator hangs the films on the light box in a specific spatial arrangement according to standard or local preferences. Determination of the orientation or reflection of an examination type, or verification of it when it is available, is beneficial to the correct display of many examination types.
In screening mammography standard 4-views of left and right breast are taken for each of the incidence directions (cranio-caudal (CC) and medio-lateral oblique (MLO)). These views are typically displayed in a mirrored fashion, such that the thorax edges or both breasts are central and touching, and the left breast image being displayed on the right and the right breast image being displayed on the left. However, because both breasts images are acquired in a similar manner, and it is in general not known which image is either corresponding to the left or the right breast, one image must be flipped before it can be positioned adjacent to the other image. In conventional screen-film imaging, X-ray opaque lead letters are radiographed simultaneously (RCC, LCC, RMLO and LMLO) with the object, and the RCC resp. RMLO films are flipped manually prior to hanging them on the right of the LCC resp. LMLO on the light box.
Digitally acquired mammograms may still be read in a conventional way by printing them on film and displaying them on a light box. Pairs of mammograms (e.g. the RCC/LCC pair and the RMLO/LMLO pair) may be printed on a single large film sheet or on two smaller sized sheets. Generally, the print margin of a hardcopy machine is adjustable, so as to minimize the non-printed portion of an image. For mammography hardcopy, the print margin corresponding to the thorax side is kept as small as possible, so that a right-left pair of images, when viewed simultaneous and in close vicinity, shows a minimal non-diagnostic viewing area in between both images. Therefore, when using a pair of small sheets to print left and right image respectively, means to identify the thorax side automatically prior to printing them is needed, because the thorax side position is generally not known or it may not be assumed to be known. Likewise, in the large film option, where both images are printed on one file sheet in order to compose the image such that the right image is touching in a mirroring manner to the left image, knowledge of the thorax side of both images is needed as well.
Digital mammography may be read on a computer display or viewing station without having resort to printed mammograms, a viewing condition known as softcopy reading. However, also here, the sub-examination types identifying right and left images may not be known at display time. Furthermore, the thorax edge orientation may not be standardized, e.g. it may either touch the left or right or the upper or lower image border. Hence, there is a need to accomplish the mirrored viewing disposition in an automated way.
Generally, a hanging protocol function enables users to arrange and display images on medical viewing stations to suit specific viewing preferences. To this purpose, the sub-examination is used to assign the sub-images pertaining to a patient study of a body part to a position in a preferred display layout of that examination type. When the image sub-type is known, hence its position in the layout is determined, the image can still be oriented in 8 different ways: it can be oriented correctly, or it can be rotated of 90, 180 or 270 degrees; in any of these four cases, the image can also be flipped around a vertical (or horizontal axis). Therefore, there is a need to derive the orientation of the image automatically, to assure viewing according to the radiological standard or local preferences.
WO 02/45437 discloses a method of determining orientations of digitized radiographic images such as mammographic images from marker images (e.g. lead marker images). The digitized radiographic images are then displayed in a predetermined orientation and order.
In the article “Two dimensional shape and texture qunatification’, Bankman I., Spisz T. S., Pavlopoulos S., Handbook of Medical Imaging, Chapter 14, pages 215-230, XP002249040, Bankman Isaac N., editor, Academic Press, 2000, a method has been described for defining the orientation of an object in an image. More particularly the direction along which an object is most elongated relative to a preferential direction such as a vertical axis is determined by calculating an angle θ in the calculation of which mathematical moment is used.